Strength and toughness of structural fibres for composite material reinforcement

Abstract

The characterization of the strength and fracture toughness of three common structural fibres, E-glass, AS4 carbon and Kevlar KM2, is presented in this work. The notched specimens were prepared by means of selective carving of individual fibres by means of the focused ion beam. A straight-fronted edge notch was introduced in a plane perpendicular to the fibre axis, with the relative notch depth being a 0 / D ≈0.1 and the notch radius at the tip approximately 50 nm. The selection of the appropriate beam current during milling operations was performed to avoid to as much as possible any microstructural changes owing to ion impingement. Both notched and un-notched fibres were submitted to uniaxial tensile tests up to failure. The strength of the un-notched fibres was characterized in terms of the Weibull statistics, whereas the residual strength of the notched fibres was used to determine their apparent toughness. To this end, the stress intensity factor of a fronted edge crack was computed by means of the finite-element method for different crack lengths. The experimental results agreed with those reported in the literature for polyacrylonitrile-based carbon fibres obtained by using similar techniques. After mechanical testing, the fracture surface of the fibres was analysed to ascertain the failure mechanisms. It was found that AS4 carbon and E-glass fibres presented the lower toughness with fracture surfaces perpendicular to the fibre axis, emanating from the notch tip. The fractured region of Kevlar KM2 fibres extended along the fibre and showed large permanent deformation, which explains their higher degree of toughness when compared with carbon and glass fibres. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Relations between intralaminar micromechanisms and translaminar fracture behavior of unidirectional FRP supported by experimental micromechanics

“…The fracture toughness of the fibers was determined from experiments carried out on notched AS4 fibers [7]. A sharp notch of size (a 0 ) to fiber diameter ratio a 0 /D ≈ 0.1 was introduced in the fiber by micro-milling using an FEI Helios NanoLab DualBeam 600i system equipped with a Focus Ion Beam (FIB), as illustrated in Figure 6(a).…”

“…where Y (a 0 /D) is known as the non-dimensional stress intensity factor and depends on the specimen geometry and the elastic properties of the material. This parameter was determined numerically using Finite Element Analysis (FEA) for a straight fronted edge crack introduced in a cylinder under uniaxial tension [7]. By means of Irwin's relation, the fracture energy of the fibers,…”

“…With the advent of experimental micromechanics techniques, it is possible to characterize the behaviors of the composite constituents with a high level of fidelity. The properties of the matrices and of the fiber/matrices interfaces can be measured in-situ using nanoindentation [4][5][6], while the fibers can be characterized by means of single-fiber tests on pristine and notched fibers [7]. Therefore, the micromechanical behaviors of the constituents can be related to the performance of the fiber reinforced material.…”

Special-purpose elements to impose Periodic Boundary Conditions for multiscale computational homogenization of composite materials with the explicit Finite Element Method

“…[4][5][6][7]) or 3D spaces (e.g. [8][9][10][11]), with input properties resulting from the experimental characterization of the composite microconstituents: fibre [12], matrix [7] and fibre/matrix interface [13]. In the case of ply architectures with higher complexity than unidirectional fibres, such as in textile composites, a subsequent homogenization step needs to be performed based on the previously computed behaviour of the unidirectional yarns, the response of the bulk resin matrix and on the topology of the Representative Unit Cell (RUC) of the fabric (e.g.…”

In Situ tensile tests to analyze the mechanical response, crack initiation, and crack propagation in single polyamide 66 fibers

“…Naito et al systematically observed the fracture surfaces of broken carbon fibers from single fibers tested in tension and identified similar fracture patterns independently of the gauge length. Other recent pioneer works have used the focused ion beam (FIB) technique so as to introduce a notch at single fiber scale on brittle fibers such as silicon carbide, carbon fibers, and so forth. This single edge notched tensile (SENT) like geometry allowed the fracture toughness of these fibers to be determined with a two‐dimensional (2D) or three‐dimensional (3D) approach.…”